How does FOREIGN KEY constraint work in SQL?
The FOREIGN KEY constraint prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the main table. The following SQL creates a FOREIGN KEY on the “PersonID” column when the “Orders” table is created:
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What is the foreign key that references an invalid table?
Foreign key ‘FK_ProductDescriptions_bacb18ce3aa67348e55d’ references invalid table ‘Product’ and “Could not create constraint or index. See previous errors.” When you create a referential constraint, you must ensure that the table and column it refers to already exist in the database.
Is the PersonID column in the Orders table a foreign key?
The “PersonID” column in the “Orders” table is a FOREIGN KEY in the “Orders” table. The FOREIGN KEY constraint prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the main table.
What is the parent table with the foreign key?
A FOREIGN KEY is a field (or collection of fields) in one table, which refers to the PRIMARY KEY in another table. The table with the foreign key is called the child table, and the table with the primary key is called the parent or reference table. Look at the following two tables:
Can a foreign key be created on an existing table?
Creating a foreign key on an existing table requires ALTER permission on the table. A foreign key constraint does not have to be linked only to a primary key constraint on another table.
Can a null compound FOREIGN KEY constraint reference?
To ensure that all values in a composite foreign key constraint are checked, specify NOT NULL on all participating columns. FOREIGN KEY constraints can only reference tables within the same database on the same server. Referential integrity between databases must be implemented through triggers.
How to get the foreign key of a row in SQL?
The values of the FOREIGN KEY columns in each row of the reference table must match the values of the corresponding primary key columns of a row in the reference table. CREATE TABLE ( column1 data_type [ (tamaño)] column2 data_type [ (tamaño)] constraint (constraint_name) FOREIGN KEY [columna1,columna2…]